HF alkylation process with internal acid regeneration

ABSTRACT

An improved process for removing polymeric by-product (ASO) from the HF alkylation acid in an HF alkylation unit used for the production of gasoline boiling range alkylate product by olefin/iso-paraffin alkylation, comprises fractionating a portion of the circulating HF alkylation acid inventory of the unit with a portion of hot alkylate product in a fractionation zone to from an overhead product comprising HF alkylation acid and water and a bottoms fraction comprising the polymeric by-product and alkylate. The bottoms fraction is sent to the isoparaffin stripper of the unit to remove trace HF alkylation acid as overhead and form a product stream of hot alkylate as a bottoms fraction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application No.60/880,950, filed 18 Jan. 2007, entitled “HF Alkylation Process withInternal Acid Regeneration”.

FIELD OF THE INVENTION

This invention relates to iso-paraffin/olefin alkylation and moreparticularly, to hydrofluoric acid (HF) alkylation. In thisspecification, the term “alkylation” will be used to refer to theiso-paraffin/olefin alkylation process used to make gasoline blendcomponents useful in aviation and motor gasolines and “HF alkylation” tothis process using hydrofluoric acid as the catalyst.

BACKGROUND OF THE INVENTION

The iso-paraffin/olefin alkylation process is widely used to manufacturea high octane quality blend component for aviation and motor gasolinewhich is also valued for its relatively low vapor pressure, lowsensitivity and, because of its freedom from aromatic components, itsenvironmental acceptability. The process typically reacts a C₂ to C₅olefin with isobutane in the presence of an acidic catalyst to producethe alkylate product.

Hydrofluoric and sulfuric acid alkylation processes share inherentdrawbacks including environmental and safety concerns, acid consumption,and sludge disposal but in spite of efforts to develop an inherentlysafe alkylation process, both processes have achieved widespreadutilization with the HF process being noted for producing a higherquality product with more favorable unit economics. Although hydrogenfluoride, or hydrofluoric acid (HF) is highly toxic and corrosive,extensive experience in its use in the refinery have shown that it canbe handled safely, provided the hazards are recognized and precautionstaken. The HF alkylation process is described in general terms in ModernPetroleum Technology, Hobson et al (Ed), Applied Science Publishers Ltd.1973, ISBN 085334 487 6. A survey of HF alkylation may be found inHandbook of Petroleum Refining Processes, Meyers, R. A. (Ed.),McGraw-Hill Professional Publishing, 2nd edition (Aug. 1, 1996), ISBN:0070417962.

In order to improve the operation of the HF alkylation process as wellas the economics of the process it is desirable to regenerate the HFalkylation acid by removing the polymeric by-product which forms duringthe alkylation reactions; this polymer, comprising polymers of differingdegrees of conjugation, is frequently referred to as “acid soluble oil”(ASO) since it is miscible with the HF acid phase. Removal of the ASO isnecessary to preserve the concentration of the acid at the high leveldesirable for good alkylation performance while removal of water isrequired in order to reduce corrosion within the unit as well as tomaintain product octane quality. Normally, the acid concentration ismaintained at 88 to 94 weight percent by the continuous or periodicaddition of fresh acid and withdrawal of spent acid with the watercontent kept in the range of 0.5 to 1.0 percent.

One previously used method for removing the polymer from the acidinventory is by internal regeneration. A small amount of acid (withpolymer and trace water) is injected into the isostripper feed line withthe large amount of alkylate and butanes feeding the tower. The acidflashes overhead and the polymer leaves with the alkylate via the bottomof the tower. There is, however, a limit on the amount of the acid inthe hydrocarbon feed to the isostripper: major corrosion problems in theisostripper overhead may occur with excess acid in the isostripper feed.Excess acid can also result in a separate acid phase in the downstreamisoparaffin recycle circuit where it mixes with olefin feed, resultingin accelerated corrosion in the tower.

When removal of water is the objective, as it is on a fairly frequentschedule, external regeneration is necessary. In the externalregeneration method, a small amount of acid (with polymer and tracewater) is injected into a separate distillation column, which isoperated in batch mode with isoparaffin used as a heating and strippingmedium. The tower separates dry acid overhead and a mixture of HF acid,water and polymer leaves as a bottoms fraction. The mixture isneutralized, and the aqueous phase is separated from polymer through aseries of separator drums. This method results in loss of HF acid andcostly use of neutralization chemicals. It is also manually intensive.

An external regeneration method of this type is described in U.S. Pat.No. 5,547,909 (Carlson). In this method, the acid phase from the settleris removed and a portion is routed to a separator column which enablesthe polymer (ASO) to be separated from the HF of the acid phase. Cooledisoparaffin is used as column reflux and heat is supplied by means ofisoparaffin introduced as stripping medium at the foot of the column.Acid, free of ASO and water is removed as overhead and is recirculatedto the settler. A portion of the bottoms fraction is recycled to ahigher level in the tower, possibly to make the separation moreeffective. While this technique may be capable of improving on theconventional external separation by attempting to get closer to anazeotropic mixture of water, acid and polymer in the bottom of thetower, (acid content of the bottoms stream about 40-50%), it still failsto achieve a satisfactory level of acid recovery and significant lossesof acid can be expected. Another external HF acid regeneration scheme isproposed in U.S. Pat. No. 6,228,650 (Moore et al) using an improvedcontrol scheme to monitor and control process operation but again, sincethe same basic regeneration scheme is used in which the heat is suppliedsolely by the iso-paraffin used for stripping, the acid regenerationremains at a level which is less than optimal. A method of stripping theHF from the alkylation acid using hydrogen is proposed in U.S. Pat. No.5,461,183 (Del Rossi et al) but this method, requiring external hydrogenis not well suited to incorporation within the non-hydrogenative HFalkylation unit.

There are therefore deficiencies in both methods of regeneration:internal regeneration in the isostripper removes polymer but isineffective for removal of water to the low levels required in theprocess while external regeneration, by contrast, is required for theremoval of water but results in an uneconomic loss of HF acid if thewater content is to be maintained at the appropriate level. Given thatthe process economics favor internal regeneration, it would be desirableto improve the operation of that practice even if it were not completelyeffective at removal of water.

We have now devised a method of HF acid regeneration which enables thepolymer to be separated from almost all of the acid, together withwater, leaving the polymer ASO with only trace acid in the alkylateproduct as a bottoms fraction. The method we have devised operates byusing the heat from a stream of hot alkylate to strip the acid, togetherwith trace levels of water, from a slipstream of alkylation acid withits polymer (and trace water) in an acid treating tower (fractionationtower) in which the acid and water are vaporized as overhead, leavingthe polymer behind as a bottoms fraction which leaves the tower withrecycled alkylate. This polymer-laden alkylate is injected into theisostripper feed to remove any trace residual acid as isostripperoverhead. The HF acid, water and light hydrocarbon vapors from thetreating tower are condensed into an overhead receiver drum, where lighthydrocarbon is separated from the acid phase. The remaining acid andtrace water is virtually polymer free and can be returned to thecirculating acid system or fed to an external regeneration unit forremoval of water, when required. The acid treating tower allows forelimination of internal regeneration as well as a reduction in externalregeneration. In addition, since the acid treating tower configurationsends only acid (and trace water) to the external regeneration unit, thecosts associated with extensive handling of polymer in externalregeneration are eliminated. Because water and acid are largely removedin the acid treating tower, the problems of isostripper overheadcorrosion from excess acid are largely eliminated.

SUMMARY OF THE INVENTION

According to the present invention, an improved process for removingpolymeric by-product (ASO) from the HF alkylation acid in an HFalkylation unit used for the production of gasoline boiling rangealkylate product by olefin/iso-paraffin alkylation, comprisesfractionating a portion of the circulating HF alkylation acid inventoryof the unit with a portion of hot alkylate product in a fractionationzone to from an overhead product comprising HF alkylation acid and waterand a bottoms fraction comprising the polymeric by-product and alkylate.The bottoms fraction is sent to the isoparaffin stripper of the unit toremove trace HF alkylation acid as overhead and form a product stream ofhot alkylate as a bottoms fraction.

In its essentials, the HF alkylation unit incorporating this recoverysystem comprises:

-   (i) an alkylation reactor with its associated acid settler having an    upper outlet for alkylate product and a lower outlet for HF    alkylation acid,-   (ii) an isostripper for separating isoparaffin from alkylate product    from the settler by fractionation to produce an isoparaffin overhead    stream at an overhead outlet and an alkylate product stream as a    bottoms fraction at a lower outlet, the isostripper having an inlet    connected to the upper outlet of the settler,-   (iii) a fractionator tower having an inlet connected to the lower    outlet of the settler to receive a slipstream portion of the HF    alkylation acid from the acid circuit,-   (iv) an inlet connected to the lower outlet of the isostripper to    receive a slipstream portion of the alkylate bottoms stream from the    isostripper.    The isostripper is also fitted with an overhead outlet for    fractionated HF alkylation acid and trace water and a bottoms outlet    for alkylate product and polymeric by-product ASO, the bottoms    outlet being connected to the inlet of the isostripper to provide    the hot alkylate for the fractionation zone as well as allowing for    withdrawal of the alkylate product stream which is taken to the    recovery and purification section.

DRAWINGS

The single FIGURE of the accompanying drawings is a simplified processschematic of an acid recovery section of an HF alkylation unit with itsassociated equipment.

DETAILED DESCRIPTION

The FIGURE shows the acid recovery section of a complete HF alkylationunit which, in addition to the components shown, includes theconventional reaction section with its reactor and feed lines and acidsettler in which the hydrocarbon phase including alkylate product isallowed to separate under gravity from the denser acid phase. Thesettler, in the conventional manner, has an inlet from the reactor (notshown) as well as an upper outlet for the alkylate product and a loweroutlet for the HF alkylation acid which circulates in the alkylationacid circuit in the conventional way, except for the slipstream taken tothe acid regeneration/recovery section described below.

The acid feed to the acid regeneration/recovery section comprises aslipstream of acid from the lower (acid) outlet of the settler; thisslipstream enters the treating section through line 10 and is circulatedby means of circulation pump 11 which forms part of the alkylation acidcircuit. A slipstream of acid passes through line 12 to the acid inlet13 of acid treater tower 14 along with hot alkylate from isostripper 15,entering the acid treater tower by way of line 16. In the tower, whichoperates as a fractionation zone, the hot alkylate from the isostripperprovides heat for flashing the acid which leaves the tower along with atrace of water as overhead through line 20. Additional heat to thefractionation tower may be providing by reboil of the bottoms fractionleaving through line 18 with the reboil entering through line 19.Alternatively, hot iso-paraffin vapor from the isostripper may beconducted from the isostripper to line 19 to add heat at this level andprovide the necessary level of vapor traffic for proper tower operation.Normally, the use of hot alkylate injection is preferred provided that asufficiently hot stream of acid-free isoparaffin is available since thereboil service requirements are quite stringent in this application.

In the ratios injected into the tower, the acid and alkylate remain astwo immiscible phases and as the acid vaporizes in the tower, thepolymer becomes a separate phase on its own, leaving the tower throughline 18 to pass to isostripper 15 in which the polymer will become fullydissolved in the larger volume of hot alkylate and iso-paraffin. Thepresence of two separate phases is acceptable in this relatively smalltower in which the acid and water from the slipstream are separated fromthe alkylate and polymer.

The overhead from the fractionation tower comprising acid plus tracewater along with any light hydrocarbon entrained in the settler and anyiso-paraffin introduced into the tower as stripping medium passesthrough line 20 to cooler 21 and then to receiver drum 22 in which thehydrocarbon phase is separated from the acid phase with the hydrocarbonbeing returned as reflux and by way of line 23 to the isostripper. Thedrum is provided with some form of oil/acid separation capability, suchas an overflow weir or other separation device. The acid from theoverhead receiver is returned through line 24 to the acid circuit bymeans of acid circulation pump 11. When water removal from acid isrequired, this acid stream can be rerouted to an external acidregenerator (not shown) by way of line 25. Since it contains no polymer,the azeotropic mixture can be neutralized and disposed of virtually freeof polymer handling problems.

The bottoms fraction from tower 14 comprising alkylate, polymer andlight hydrocarbon from receiver drum 22, is passed to isostripper 15 andenters the isostripper (itself a fractionator) at a level downstream ofthe preheat exchanger since, without the need for internal acidregeneration, a high level of acid solubility is present to maintain thebottoms stream trace acid in solution. The isostripper separates out thealkylate as bottoms, leaving the isostripper through line 30 and passingto the product recovery section (not shown) through line 34 via heatexchanger 31 which provides heat for the incoming alkylate-containingstream from the settler in line 32; the slipstream of hot alkylate fromthe isostripper is taken out into line 16 to the treater tower asdescribed above. Isoparaffin passes out through line 33.

The acid treatment tower and associated equipment which comes intocontact with the acid-containing streams must be built of acid-resistantalloy materials such as Monel™ metal, since the acid phase can be morecorrosive when heated. This, however, is economically favorable comparedto the use of corrosion-resistant alloys in the isostripper overheadwhich is a larger item of equipment. The acid treatment tower operatesat a higher temperature than the isostripper so that the composition ofthe overhead can approach azeotropic in a way that is not possible inthe isostripper. The number of stages of separation required in thetower will be dependent on the desired degree of separation betweenhydrocarbon (alkylate+polymer) and acid. In the simplest case, a flashdrum can be used, which will send more hydrocarbon overhead.Alternatively, a refluxed tower with many stages minimizes the amount ofhydrocarbon going overhead. In either case, some butane and lightalkylate will end up condensing in the overhead receiver-drum. Becausethis fraction will contain dissolved acid, it will need to be reinjectedback into the isostripper tower via line 23 for removal of the acid atthat point.

The remainder of the HF alkylation process including identities and feedhydrocarbons, temperatures and equipment items can be conventional intype. The olefin feed will typically be C₂-C₄ olefins with preferencebeing given to butylenes although propylene and ethylene may also beused with corresponding changes in product properties and in theappropriate process conditions, as is known. The isoparaffin mostfavored for use in the HF alkylation process is isobutane. The alkylateproduct may be treated in the conventional way in the product recoveryand purification section, e.g. by caustic neutralization to form thedesired gasoline blend component.

The process may be operated with a vapor suppressant additive to improvesafety margins in the event of an uncontrolled acid release from theunit. The vapor suppressant additives normally contemplated are thosewhich reduce the volatility of the HF acid. Compounds of this type whichhave been proposed include organic sulfones, ammonia, amines such as thelower alkylamines (methyl to pentyl), pyridine, alkylpyridines,picoline, melamine, hexmethylenetetramine. A number of differentsulfones have been proposed for this purpose but the one generallypreferred is sulfolane (tetramethylenesulfone) with 3-methylsulfolaneand 24-dimethylsulfolane also being suitable. A more detaileddescription of vapor suppressant additives of this type is given in U.S.Pat. No. 6,114,593 to which reference is made for this description. Whena vapor suppressant additive is used the process is often referred to asmodified HF alkylation (MHF).

1. A process for removing polymeric by-product ASO (acid soluble oil)from the circulating HF alkylation acid in an HF alkylation unit usedfor the production of gasoline boiling range alkylate product byalkylation of an isoparaffin with an olefin, which comprises: alkylatingan isoparaffin with an olefin in the presence of HF alkylation catalystin an alkylation reactor, subjecting the HF alkylation reaction productfrom the reactor to separation in an acid settler from which a streamcomprising a portion of the HF alkylation acid containing ASO iswithdrawn, mixing the withdrawn HF alkylation acid stream with a portionof hot alkylate product from an isoparaffin stripper of the unit,fractionating the withdrawn HF alkylation acid stream in a fractionationzone having vertically spaced lower and upper ends with the lower endmaintained at a higher temperature than the upper end by heat suppliedto the lower end by external reboil of a bottoms stream obtained fromthe lower end, to form an overhead product comprising HF alkylation acidand water and a bottoms fraction comprising the polymeric by-product andalkylate and fractionating the resulting bottoms fraction in theisoparaffin stripper of the unit to remove trace HF alkylation acid asoverhead and form a stream of hot alkylate product and polymericby-product as a bottoms fraction in the isoparaffin stripper.
 2. Aprocess according to claim 1 in which HF alkylation acid and water inthe overhead of the fractionation zone also comprises light hydrocarbonand the HF alkylation acid and water in this overhead are separated fromthe light hydrocarbon and sent to the HF alkylation acid circulating inthe unit.
 3. A process according to claim 2 in which the lighthydrocarbon separated from the HF alkylation acid is sent to theisoparaffin stripper with the alkylate product.